1. Field of the Invention
The present invention relates to an exhaust emission control catalyst apparatus in an internal combustion engine, and more particularly, to a technique for enhancing a conversion performance of HC (hydrocarbon) by a HC adsorption catalyst when it is cold.
2. Description of Related Art
Conventionally, there is a known exhaust emission control apparatus in which a HC (hydrocarbon) adsorption material is interposed in an exhaust passage of an internal combustion engine, the HC in exhaust is adsorbed by the HC adsorption material when it is cold, the HC is eliminated from the HC adsorption material after warming up of the engine, the eliminated HC is purified by three way catalytic converter or oxidizing catalyst disposed at the exhaust downstream of the HC adsorption material (see Japanese Patent Application Laid-open No. 5-59942 and the like).
That is, in this exhaust emission control apparatus, utilizing property of the HC adsorption material that the HC adsorption material adsorbs HC when temperature is low and eliminates HC when temperature rises to a fixed temperature, cold HC is adsorbed by the HC adsorption material, and when the exhaust temperature rises to the fixed value, the HC eliminated from the HC adsorption material is purified by the three way catalytic converter.
However, the exhaust emission control apparatus having the above described conventional HC adsorption material has the following problem.
That is, temperature rise of the HC adsorption material located upstream the exhaust passage is faster than that of the three way catalytic converter located downstream.
Therefore, when the elimination of the HC from the HC adsorption material is started, if a rear three way catalytic converter or oxidizing catalyst does not reach the activating temperature, there is an unfavorable possibility that the eliminated HC is not converted and is discharged, and discharge of the HC when it is cold can not be suppressed.
Further, another prior art includes the HC adsorption material upstream the exhaust passage, and exhaust emission catalyst for purifying the HC downstream the exhaust passage (Japanese Patent Application Laid-open No. 8-284646). In this prior art, by controlling heat capacity of the exhaust emission catalyst to a value smaller than heat capacity of the HC adsorption material, temperature rise of the exhaust emission catalyst located downstream is made faster than that of the HC adsorption material.
In this case, if the emission control catalyst can reach the activating temperature before the upstream end of the HC adsorption material which is most liable to receive the heat of the exhaust gas reaches the elimination temperature, it is possible to mostly purify the HC eliminated from the HC adsorption material. However, it is practically difficult to increase the difference in heat capacity to such an extent.
Another prior art is provided with HC adsorption catalyst in which a layer of catalyst for purifying HC is formed above a layer of HC adsorption material (Japanese Patent Application Laid-open No.8-224449). In this prior art, when the HC adsorbed by the HC adsorption material layer is eliminated, the HC passes the catalyst layer without fail, and temperature of the upper catalyst layer which directly contact with the exhaust gas rises faster than the lower HC adsorption material layer.
In this case, if the upper catalyst layer reaches the complete activating temperature when the lower HC adsorption material reaches the elimination temperature, it is possible to mostly purify the HC. However, since the HC elimination temperature of zeolite which is used as the HC adsorption material at the present is much lower than the activating temperature, when the HC adsorption material layer reaches the elimination temperature, the catalyst layer does not reach the complete activating state and therefore, it is not possible to purify all the eliminated HC by the catalyst layer.
In such a HC adsorption catalyst also, since the temperature rises from the upstream side which is most liable to receive heat of the exhaust gas, the elimination of the HC is started from the upstream side with the temperature rise. Among the eliminated HC, a portion thereof which is not purified by the catalyst layer is again adsorbed by the downstream side whose temperature has not risen yet and therefore, such HC should not be discharged to open air as it is. However, when a portion of the HC adsorption material layer in the vicinity of its downstream side also reaches the elimination temperature, the HC which is eliminated from the vicinity of the downstream side and which is not purified by the catalyst layer is discharged to open air as it is.